Various structures have been developed to supply r.f. fields from devices outside of a vacuum chamber to excite a gas in a plasma processor to a plasma state. The r.f. fields have been derived from electric field sources including capacitive electrodes, electromagnetic field sources including electron cyclotron resonators, and induction, i.e., magnetic, field sources including coils. The excited plasma interacts with a workpiece in the chamber to etch the workpiece or to deposit material on it The workpiece can be a semiconductor wafer having a planar circular surface or a solid dielectric, e.g., a rectangular glass substrate used in flat panel displays, or a metal plate.
A processor for treating workpieces with an inductively coupled planar plasma (ICPP) source is disclosed, inter alia, by Ogle, U.S. Pat. No. 4,948,458, commonly assigned with the present invention. In Ogle, the magnetic field that excites the plasma is derived from a planar coil positioned on or adjacent to a single planar dielectric window that extends in a direction generally parallel to the workpiece planar surface being processed. The coil is connected to be responsive to an r.f. source having a frequency in the range of 1 to 100 MHz (typically 13.56 MHz) and coupled to the coil by an impedance matching network. The coil is configured as a planar linear spiral having external and internal terminals connected to be responsive to the r.f. source. Coultas et al., U.S. Pat. No. 5,304,279 discloses a similar device employing plasma confinement using permanent magnets in combination with the planar spiral coil.
Cuomo et al., U.S. Pat. No. 5,433,812 and Ogle, U.S. Pat. No. 5,277,751 disclose variations of the aforementioned processors wherein the planar spiral coil is replaced by a solenoidal coil. The solenoidal coil is wound on a dielectric mandrel or the like and includes plural helical-like turns, a portion of which extend along the dielectric window surface. The remainder of the coil extends above the dielectric window. Opposite ends of the solenoidal coil are connected to an r.f. excitation source.
These inductive sources excite the plasma by heating electrons in the plasma region near the vacuum side of the dielectric window by oscillating inductive fields produced by the coil and coupled through the dielectric window. Inductive currents which heat the plasma electrons are derived from the r.f. magnetic fields produced by r.f. currents in the planar coil. The spatial distribution of the magnetic field is a function of the sum of the fields produced by each of the turns of the coil. The field produced by each of the turns is a function of the magnitude of r.f. current in each turn. For the spiral design disclosed by the Ogle '458 patent, the r.f. currents in the spiral coil are distributed to produce a ring shaped region where power is absorbed by the plasma. The ring shaped region abuts the vacuum side of the dielectric window. At low pressures, in the 1.0 to 10 mTorr range, diffusion of the plasma from the ring shaped region produces a plasma density peak in a central portion of the chamber, along a chamber center line away from the window. At intermediate pressure ranges, in the 10 to 100 mTorr range, gas phase collisions of electrons, ions, and neutrons in the plasma prevent substantial diffusion of the plasma charged particles outside of the annular region. As a result there is a relatively high plasma flux in a ring like region of the workpiece but low plasma fluxes in the center and peripheral workpiece portions. Hence, there are substantially large plasma flux variations between the ring and the volumes inside and outside of the ring.
Chen et al., U.S. Pat. No. 5,226,967 considers the adverse effects of reduced plasma density at radial regions removed from the center of a planar spiral coil. In Chen '967, the strength of magnetic fields generated by the planar coil and coupled to the plasma decreases along the chamber center line. The decrease is provided by increasing the thickness of the dielectric window center portion, relative to the thickness of other regions of the window. At pressures up to about 20 mTorr, the increased thickness of the solid dielectric material extending into the plasma shifts the ring shaped region for r.f. power absorption to a larger radius. The shift of the ring shaped region position changes the diffusion characteristics of this plasma generation region so diffusion is more uniform across the entire processed substrate surface diameter, particularly at the peripheral portion of the substrate.
In the device of the '967 patent, an electromagnetic shield that supports the dielectric window decreases the plasma flux close to the center of the coil. This plasma flux density reduction occurs because the shield decreases coupling between the coil and an electromagnetic field resulting from the r.f. current applied to the coil. The magnetic fields produced by the largest diameter turn of the planar source frequently induce r.f. currents in the electromagnetic shield which supports the window, if the shield and largest turn are sufficiently close to each other. Power coupled to the shield results in (1) a decrease in the coupling efficiency of the r.f. excitation of the plasma and (2) a shift of the ring shaped power absorption region to a smaller diameter region since the magnetic field produced by the largest diameter turn of the coil does not couple as much magnetic flux to the plasma as the inner turns. Substantial uniformity up to about 20 mTorr occurs as a result of diffusion of charged particles into the region below the center of the coil, where the window is thickest. However, as pressure increases above about 20 mTorr, where charged particular diffusion decreases appreciably, the plasma flux beneath the center of the coil, where the r.f. excitation is small, decreases relative to the flux in the other regions beneath the coil. Hence, there is non-uniform plasma flux on different portions of the workpiece.
The ring shaped region over which a planar coil couples r.f. power to the plasma can be shifted to larger diameters by removing the inner turns of the planar spiral. Fukusawa et al. in an article entitled "RF Self-Bias Characteristics in Inductively Coupled Plasma," Japanese Journal of Applied Physics, Vol. 32 (1993), pages 6076-6079, Part 1, No. 12(B), December 1993, discloses a single turn planar spiral coil for exciting gases in a plasma processor to a plasma condition. The disclosed coil has inner and outer dimensions of 120 and 160 mm and is 0.5 mm thick and is located in the vicinity of the periphery of a dielectric plate which serves as the top of the vacuum vessel. The ostensible purpose of the one turn coil is to produce a uniform plasma density radial distribution. However, as illustrated in FIG. 6 of Fukusawa et al., the plasma density is not particularly uniform even at a relatively low plasma processor pressure of 10 milliTorr, particularly for the types of r.f. excitation power which are required for plasma etching, in the 1 kilowatt range. For 1 kilowatt r.f. excitation of the single turn source, disclosed by Fukusawa et al., the plasma produces a substantial plasma density peak (of 7.5.times.10.sup.11 ions cm.sup.-3), at a position approximately 4.0 centimeters from the center of the chamber. This peak occurs only on one side of the center axis, creating a significant non-radial asymmetry in the spatial distribution of the plasma. The one-turn spiral disclosed by Fukusawa et al. results in a shift in the coupling region to a larger diameter relative to the full spiral disclosed by Ogle in the '458 patent. The Fukusawa et al. devices also exhibit a nonradial asymmetry which equals or exceeds the radial nonuniformity in the spatial distribution of the plasma density generated by a full spiral. The range of pressures over which the one turn coil can be operated is also limited since this coil relies on diffusion to shift plasma from the ring shaped plasma generation region near the periphery of the chamber to the center of the chamber. At pressures above 10 milliTorr, collisions of charged and uncharged particles in the plasma result in a severe decrease in plasma density in the center region of the chamber.
The Invention
In accordance with one aspect of the present invention, a vacuum plasma processor has a coil with plural arcuate turns for exciting gas in the processor to a plasma state in response to r.f. energization of the coil. The coil includes interior and exterior radially displaced segments and is arranged so the magnetic flux derived from outer segment is greater than the magnetic flux derived from the center portion of the coil. The spatial distribution of the magnetic flux derived from the improved coil is arranged to substantially eliminate nonradial asymmetries in the spatial distribution of the plasma compositions (or composition) which interacts) with the workpiece. The spatial arrangement of the magnetic flux of the coil is able to excite spatially uniform plasma fluxes for pressures between 1 and 100 mTorr. The vacuum chamber is optimized to reduce coupling of magnetic flux from peripheral portions of the coil to an electromagnetic shield.
The coil has sufficient length at the frequency (i.e., the wavelength) of the r.f. source to generate a significant standing wave pattern along the length of the coil due to transmission line effects. Because of the transmission the effects at least one r.f. current maximum exists in the coil at some point along the geometric length of the coil. The magnetic flux (which in turn generates the induction field for exciting the plasma) produced by each coil segment is proportional to the magnitude of the r.f. current occurring in the segment. As a result, a single loop connected to an r.f. supply is expected to produce a nonradial maximum in the plasma density at a position corresponding to the location of the coil r.f. current maximum. This type of nonradial maximum is seen in the Fukusawa et al. data. For a coil with plural ring segments, the plasma density spatial distribution is a function of both the geometries of the segments and the level of r.f. current occurring in each segment. For the multiple element spiral coil disclosed by the Ogle '458 patent, the spatial average of the magnetic flux produces a plasma with ring shaped plasma generation region near the center of the vacuum side of the dielectric window. Only a small degree of nonradial asymmetry is produced by the r.f. current maximum occurring in one portion of the spiral.
In the improved coil of the present invention, the interior and exterior coil segments are positioned and arranged so the magnetic fluxes from different regions of at least some pairs of adjacent coil turns are additive. Because these different regions are adjacent each other, the magnetic fluxes from them add and average to a value which produces a uniform plasma flux across the workpiece processed surface. The elements of the coil are arranged to produce a greater degree of magnetic flux from the outer turns relative to the magnetic flux produced by the inner turns. By providing maximum magnetic flux in the outer turns and minimum flux in the smaller diameter inner turns, the outer diameter of the ring shaped plasma generation region is extended. The proper choice of diameter size for each of these segments results in a planar coil which produces a plasma generation region which diffuses to and uniformly interacts with the workpiece processed surface. The diameters of these ring segments are arranged to produce this uniform flux across the workpiece processed surface over a wider range of operating pressures, typically from 1 milliTorr to 100 milliTorr.
The production of a uniform plasma flux on the workpiece processed surface requires correct selection of the aspect ratio of the chamber, i.e., the ratio of cylindrical chamber diameter to the distance between the workpiece processed surface and the bottom of the dielectric plate (the top of the chamber). At the correct aspect ratio, the ring shaped plasma at the top of the chamber diffuses to the workpiece surface so there is a spatially uniform plasma flux on the workpiece processed surface. To this end, in one preferred embodiment, the chamber upper surface above which the coil is mounted consists of only a 14.7 inch diameter circular quartz plate window with a uniform thickness of 0.8 inches. The plasma chamber is a cylindrical vessel with a metal wall having a 14.0 inch inner diameter. The planar coil outer diameter is about 12 inches. The resulting one inch gap between the bottom face of the coil and the top face of the chamber (defined by the bottom face of the quartz plate) prevents the magnetic flux in the outermost turn of the coil from being significantly coupled to the metal chamber wall. A metal electromagnetic shield enclosure which surrounds the planar coil is located beyond the chamber wall outer diameter so magnetic flux in the outermost turn of the coil is not significantly coupled to the shield enclosure. The cylindrical shape of the plasma vessel assists in producing a uniform plasma flux on the workpiece processed surface. The spacing of the workpiece from the plasma generation region is optimized to produce a uniform flux on the workpice processed surface over a wide range of operating pressures. For the coil and chamber diameters listed above, the spacing between the vacuum side of the dielectric plate and the upper, processed workpiece surface is about 4.7 inches.
Because of the transmission line effects of the coil, a spatially averaged r.f. current is produced in the coil to provide uniform plasma excitation over a wide range of operating pressures. To provide this degree or capacitive coupling without degrading the overall coupling of the coil to the plasma, the coil r.f voltage spatial distribution is optimized. In one embodiment, capacitive coupling optimization is primarily accomplished by exciting the coil so it has capacitive coupling peaks (i.e. the points with the highest r.f. voltages) at terminals of the coil in the coil center. These terminals are connected to the r.f. excitation source.
The plasma chamber is designed so there is a substantially uniform spatial plasma flux on the workpiece. In one preferred embodiment, the chamber upper surface consists of only a 14.7 inch diameter circular quartz plate with a uniform thickness of 0.8 inches. The plasma chamber is a cylindrical vessel with a metal wall having a 14.0 inch inner diameter. The outer diameter of the planar coil is 12 inches. The resulting one inch gap between the bottom face of the coil and the top face of the chamber (defined by the bottom face of the quartz plate) prevents the magnetic flux in the outermost turn of the coil from being significantly coupled to the metal chamber wall. A metal electromagnetic shield enclosure which surrounds the planar coil is located beyond the chamber wall outer diameter so magnetic flux in the outermost turn of the coil is not significantly coupled to the shield enclosure. The cylindrical shape of the plasma vessel assists in producing a uniform plasma flux on the workpiece processed surface. The spacing of the workpiece from the plasma generation region is optimized to produce a uniform flux on the workpiece processed surface over a wide range of operating pressures. For the coil and chamber diameters listed above, the spacing between the vacuum side of the dielectric plate and the upper, processed workpiece surface is 4.7 inches.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, especially when taken in conjunction with the accompanying drawings.